1. Field of the Invention
The present invention relates to an active matrix substrate to construct a display device in combination with a display medium such as liquid crystal.
2. Description of the Prior Art
An active matrix display device includes a substrate and pixel electrodes arranged in a matrix on the substrate, each pixel electrode having active elements such as a thin film transistor (hereinafter called the "TFT"). The pixel electrodes are charged when the TFTs are on. Until all the pixel electrodes for one frame become on, the electric charge is held. The liquid crystal sealed between the charged pixel electrodes and counter electrodes thereto is optically modulated and displayed. The active matrix display device has advantages of being thin, light-weight, and consuming less electric power, and is in wide use for flat display panels.
Referring to FIG. 5, an example of the conventional active matrix substrate will be described:
Pixel electrodes 2 are arranged in a matrix on a glass substrate, and each pixel electrode is provided with a TFT 3 as an active element. Gate buses 4 and source buses 5 are disposed between the pixel electrodes 2 so as to drive the TFTs 3. In order to increase the capacity of pixel electrodes 2 for holding electric charge during a period for one frame, additional capacitance electrodes 15 are disposed under the pixel electrodes 2. Alternatively, the additional capacitances can share the gate buses 4.
The fabrication of such active matrix substrates requires a series of complex processes which consist of fabricating TFTs and etching. The TFTs 3 and the additional capacitance electrodes 15 must be provided to each of such a huge number of pixel electrodes as 100,000 or more without scarifying or losing the electric characteristics. To achieve this, a precision process control is required to maintain the electric characteristics of each pixel electrode.
However, even if the manufacturing process is precisely controlled, leakage is likely to occur between the additional capacitance electrodes and the pixel electrodes, thereby resulting in defective pixel electrodes. The defective pixel electrodes are detected and repaired at the time of quality check. The defective pixel electrodes are repaired by removing a leak occurring portion of the pixel electrode 2 on the additional capacitance electrode 15. The defective portion of the pixel electrode is cut with a suitable trimmer, or supersonic wave cutter, etc. The cutting involves difficulty because the length to be cut is equal to the length of a side of the pixel electrode, and in addition, takes a long time.
In order to facilitate the cutting of a defective pixel electrode, the active matrix substrate shown in FIG. 6 is used. The substrate of FIG. 6 is the same as the one shown in FIG. 5 except that the pixel electrode 2 is divided into the additional capacitance portion 2b and the non-additional capacitance portion 2a, and that a conductive film 30 is disposed so as to electrically connect the additional capacitance portion 2b and the non-additional capacitance portion 2a. FIG. 7 is a sectional view taken line P-P of FIG. 6 showing the vicinity of the TFT 3. FIG. 8 is a sectional view taken line Q-Q of the FIG. 6.
In this type of active matrix, the gate buses 4, gate electrodes 7 branched from the gate buses 4 and an additional capacitance electrode 15 are formed on a glass substrate 6, and an insulating film 8 is disposed to cover them by anodic oxidation. Finally the gate insulating film 9 made by the plasma CVD is formed over the whole surface of the substrate 6. A semi-conductive film 10 is formed above the gate electrode 7 over the insulating films 8 and 9. On the semi-conductive film 10, a first source electrode 11 and a second source electrode 12, which are connected to the source bus 5, and a first drain electrode 13 and a second drain electrode 14, which supply electric charges to the pixel electrodes 2, are respectively stacked. As shown in FIG. 6, the pixel electrode 2 is divided into an additional capacitance portion 2b located in opposition to the additional capacitance electrode 15 and other non-additional capacitance portions 2a.
A conductive film 30 is formed on the gate insulating film at the same time as when the source bus 5, the second source electrode 12 and the second drain electrode 14 are formed. The additional capacitance portion 2b and the non-additional capacitance portion 2a of the pixel electrode 2 are formed on the gate insulation film 9, and the edges of these portions 2b and 2a are formed on the conductive films 30.
An advantage of this construction is that a defective pixel electrode can be easily repaired by cutting the conductive film 30 with a laser beam when leakage occurs between the additional capacitance portion 2b and the additional capacitance electrode 15 composing the pixel electrode 2 so that the additional capacitance portion 2b and the non-additional capacitance portion 2a are electrically disconnected. The width of the conductive film 30 is so short that the conductive film 30 may be easily cut.
In order to simplify the manufacturing process, the conductive film 30 and the source bus wiring 5 are formed on the active matrix substrate at the same time. As a result, the conductive film 30 has a thickness of about 200 to 400 nm which is the same thickness with that of the source bus wiring 5. In contrast, since the additional capacitance portion 2b and the non-additional capacitance portion 2a, both of which are made of transparent conductive film, are formed to the thickness of about 100 nm, a step cut portion 40 may occur in the additional capacitance portion 2b and the non-additional capacitance portion 2a formed on the step portion made by the thick conductive film 30 as shown in FIG. 9. Moreover, when patterns of the additional capacitance portion 2b and non-additional capacitance portion 2a are formed by etching, the etchant is likely to penetrate into the step portion, so that the step cut portions 40 may detrimentally spread. The increased number of step cut portions result in poor characteristics for holding electric charges stored in the pixel electrode 2.